Oxidation-reduction reactions are among the most important in the biosphere. The long term goal of this research is to obtain a better understanding of metalloenzyme redox catalysis, with an emphasis on establishing the correlations between metal site structure and catalytic function. Motivation for this work lies in the observation that biological macromolecules often show selectivities and sensitivities which are unrivaled by conventional chemical systems, and in the expectation that an understanding of the correlation between structure and function will ultimately permit rational control of enzymatic reactivity. The specific aims of the current proposal are to characterize in detail the structure and reactivity of the Mn site(s) in Mn redox enzymes. Manganese enzymes are substantially less well characterized than their Fe and Cu counterparts. However, there is increasing recognition of the importance of Mn enzymes in key biological processes. The first part of the proposal focuses on superoxide dismutase and catalase, manganese enzymes which protect against oxidative damage by catalyzing the decomposition of reduced oxygen intermediates. The second part examines the system that, in a sense, is responsible for oxidative stress. The latter is the photosynthetic oxygen evolving complex, which catalyzes the oxidation of water to dioxygen during photosynthesis and which is responsible for production of most of the oxygen in the atmosphere. The underlying principle that motivates this proposal is the recognition that by determining the structure of the metal active-site, and by comparing the site-structure for different enzymatic derivatives, it is possible to learn a substantial amount about the functioning of an enzyme. A more thorough understanding of biological oxygen metabolism will be valuable in designing homogeneous catalysts which mimic biological reactions. Moreover, a detailed understanding of the mechanisms by which the biological systems function should eventually have numerous practical applications including, for example, the rational design of inhibitors for specific metabolic processes and the selective modification of metalloproteins so as to alter their reactivity. Although the immediate focus of the present proposal is Mn enzymes, the proposed experiments will have broader importance within bioinorganic chemistry. There is a growing recognition that there are substantial similarities, both in terms of reactivity and in terms of structure, between the non-heme iron proteins and their Mn analogs. The proposed experiments will allow more complete comparison between Fe and Mn sites in these proteins.